Housecleaning: Use walrus operator in lock (#6554)

Co-authored-by: Keith Burzinski <kbx81x@gmail.com>

esphome/components/lock/__init__.py

@@ -57,8 +57,8 @@ async def setup_lock_core_(var, config):
         trigger = cg.new_Pvariable(conf[CONF_TRIGGER_ID], var)
         await automation.build_automation(trigger, [], conf)
 
-    if CONF_MQTT_ID in config:
-        mqtt_ = cg.new_Pvariable(config[CONF_MQTT_ID], var)
+    if mqtt_id := config.get(CONF_MQTT_ID):
+        mqtt_ = cg.new_Pvariable(mqtt_id, var)
         await mqtt.register_mqtt_component(mqtt_, config)